Audio production for television, video, film, and recorded music sales is a large and growing enterprise, and is a basic element in much of the entertainment industry. Audio equipment, such as amplifiers, mixers, synthesizers, and the like, are under constant development as the basis for new products for audio engineers. As has been true in many other technical fields, computerization has become important in audio equipment, and many new products include some form of computerization.
Computerized equipment is becoming more and more important as the basis of technical advances in the audio control industry to provide ability to mix and process more sophisticated and more voluminous audio input, and to provide more flexibility in output. Computerization is also seen as a requirement for cost-effective competition. Manual instruments, systems, and techniques are, by comparison, increasingly more expensive to use.
In the very early days of music recording, after recording media became available, the approach to making a recording was quite simple. One merely turned on the recorder, which included an input device such as a horn or a microphone, while the audio phenomenon to be recorded was taking place, and the instrument recorded, as best it could, whatever audio was intercepted by the input device. The results were understandably somewhat crude.
As more advanced equipment became available, and a better understanding of audio principles was developed, a number of reasons for "poor" or incomplete recording have been discovered. A very important discovery involved the concept of "dynamic range". Dynamic range is defined as the difference between the lowest level and the highest level a system or device can effectively handle. The concept is applicable to both "sending" or transmitting equipment, and to receiving equipment.
The human ear, for example, has a dynamic range of about 120 dB. All of this range, though, is not always useful, and there is always "noise" to a level below which lower level useful signals are mixed, and become not readily discernable over the noise. The useful dynamic range of the human ear, also called signal to noise ratio (S/N ratio), is about 90 decibels.
Staying with the idea of this useful dynamic range, the useful range (S/N ratio) of a symphony orchestra is about 80-90 dB. The human ear, then, is quite capable of receiving and utilizing the full dynamic range of a symphony orchestra. The difficulty arises when the useful range of various audio devices and instruments is considered. For example, the useful range of a state-of-the-art compact disk is about 96 dB, of a high quality microphone about 75 dB, of a digital tape recorder about 96 dB, of an analog tape recorder about 65-70 dB, and of an FM transmission about 60 dB. A normal telephone line has a typical useful range of about 45 dB.
It is easy to see from the above information that most audio devices, even state-of-the-art, high quality devices, have a useful range less than that of the human ear, and in some cases, less than the range of an orchestra, or some other audio event, which one might want to record.
The apparent problem, then, is that one cannot always record the full range of an audio phenomenon to render it essentially unchanged at a later time to the human ear, without having some means of reducing, or compressing, the audio input to a range within the capability of the audio instrument intended to be used, and then expanding the recorded phenomena when it is later played, back to its original audio range, or even more.
Over the years of development of audio equipment, such as receiving, recording, and playback equipment, devices have been developed to alter level in an audio operation performed by an audio instrument. Such devices are generally termed amplifiers, although they may also act to attenuate signals. Early amplifiers were fixed-gain devices, and variable-gain devices were developed later. Among variable-gain amplifiers there are compressors, gates, limiters, and expanders.
Briefly, a compressor is a device that receives an audio signal having a dynamic range of R.sub.1 and puts out (or transmits) the signal with a new dynamic range of R.sub.2, where R.sub.2 &lt;R.sub.1. The ratio of R.sub.1 to R.sub.2 is called the compression ratio of the compressor.
A good example of use of a compressor is in a hypothetical case involving a desire to record a signal having a dynamic range broader than the dynamic range of the recording medium desired to use. A compressor may be employed to receive the signal, compress its dynamic range, and pass it on the recording medium with a new range within the range of the recorder. An expander might be used at a later time to receive the signal from the recording medium and to restore the original dynamic range before the signal is transmitted to an output device, such as a speaker (or a series of speakers). Both compressors and expanders may have a threshold, such that the compressing or expanding effect is only active above (for a compressor) or below (for an expander) a certain dB level.
A gate is also often called a noise gate, and is an expander set to sharply attenuate any input that falls below threshold. A gate is used to "clean up" noise. A limiter is a compressor whose compression ratio, defined as the ratio of input to output dynamic range, is equal or greater than about ten to one. A limiter is typically used at the high end of an input range to convert input to essentially constant output level.
Beyond the problems of varying dynamic range to accommodate the range limitations of different devices and systems, dynamic range control is also useful for altering audio signals to create special audio effects to enhance the perceived audio quality of a signal.
Audio level dynamic range control is arguably one of the most important and essential capabilities for any kind of audio equipment, all the way from simple devices to complicated mixer boards. Yet audio level dynamics processors have always been, and are still, before the present invention, implemented by such devices as rotary and slide potentiometers and switches typically hard-wired to the devices they control, and visual feedback is limited to lights, LED's, meters, and simple displays on monitor screens. The use of potentiometers and the like to control amplifiers for audio dynamics is quite limiting due to temporal factors, as well as wide variance in characteristics of such devices. Moreover, it is difficult for a sound engineer to know what he or she has done until it is too late to make a reasonable correction. Another drawback is that there is no way to save a successful or desirable "gain riding" configuration to recall and use at a later time,
Some of the information in the background section of this specification is taken from a book titled "Sound Recording Handbook" by John M. Woram and published by Howard W. Sams and Co., a division of Macmillan, Inc., Copyright 1989 by John M. Woram. The concepts and devices typically used in audio dynamics are the subject of Chapter 8, which is a good basic treatment of the subject matter.
What is needed is a new way to set audio dynamics for variable gain amplifiers with visual feedback to provide a visual interpretation of how a signal is being processed, to control with high resolution and accuracy, and to be able to return to successful characteristics and parameters as a starting point for new applications.